Multi-Wire Branch Circuits

A multi-wire branch circuit in a residential dwelling contains two hot wires of different phases (A and B phase) and share one neutral wire as return current. When a three wire NM cable is used for this type of circuit:

The black wire is usually the A phase line voltage.
The red wire is usually the B phase line voltage.
The white wire is the neutral, providing return current for both of the A and B phase line voltage.
The bare copper wire is the ground.

See example diagrams below. Outlet ground connection diagram shown separately.

Multi-Wire Branch Circuit Correct Wiring

The neutral wires are connected together before connecting to the terminals on the left outlet. The NEC does not require the hot (black) wires to be connected together before connecting to the terminals on the left outlet.

Multi-Wire Branch Circuit Wiring Diagrams

Open Neutral Multi-Wire Circuit

It is preferred that the hot (black) wires be connected together before connecting to the terminals on the left outlet, even though it is not required by the NEC.

Multi-Wire Branch Circuit Preferred Wiring Diagram

Multi-Wire Branch Circuit Incorrect Wiring

The neutral wires are not connected together before connecting to the outlet on the left. The two left outlets do not meet the NEC for a multi-wire branch circuit. If the neutral wires are disconnected with B phase power turned on, the neutral wire from the B phase outlet has line voltage.

Multi-wire Branch Circuit Correct Wiring Diagram

Multi-wire Branch Circuit Preferred Wiring Diagram

Multi-wire Branch Circuit Incorrect Wiring Diagram